Ziegler-Natta catalysts are widely used to produce polyethylene and copolymers thereof. There are many varieties and methods for making Ziegler-Natta catalysts, such as depositing a titanium complex on a solid support such as magnesium chloride and/or silica. Ziegler-Natta catalysts are fairly inexpensive to produce and usually generate polymer products at high levels of productivity.
Typical Ziegler-Natta products have a molecular weight distribution (MWD) greater than about 2.0, more commonly greater than about 3.0, and a melt flow ratio (MFR) defined as I21/I2 ranging from about 24 to about 28. Polyethylene films produced from Ziegler-Natta catalyzed resins are known for excellent toughness and tear properties. Processing properties of polyethylene produced using Ziegler-Natta catalysts are also affected by long-chain branching. For example, long-chain branches, even at very low concentrations, have a strong effect on the polymer melt behavior and, thereby, the processing properties.
There is a need, therefore, for the ability to control the amount of long-chain branching that occurs during the production of polyethylene resins using Ziegler-Natta catalysts.